In general, the photographic material used for photographing human internal organs, etc., by X-rays, i.e., photosensitive material for X-ray use, consists either of indirect photography X-ray film wherein visible images on a fluorescent plate produced by X-rays are photographed with an optical lens, or direct photography X-ray film where no optical lens is used and the fluorescence produced by irradiation with the X-rays is directly recorded on film (referred to below as "X-ray film for direct use").
X-ray film usually has at least one layer of photosensitive silver halide emulsion on each side of a transparent support.
In general, when forming an image on X-ray film for direct use by X-ray irradiation, the film is placed between fluorescent screens and irradiated with X-rays. The X-ray energy absorbed by the fluorescent screens on both sides of the film emits a blue-green fluorescence, to which the film is photosensitive. As a result, an X-ray image is formed.
An image may be formed by exposure of the photosensitive material to the X-ray energy. However, the film is overwhelmingly more sensitive to the blue-green fluorescence.
Thus when forming images directly on X-ray film, X-ray energy can be efficiently used by the concurrent use of fluorescent screens. Thus, the radiation exposure dose is reduced.
On the other hand, when using fluorescent screens, the sharpness of the image deteriorates.
This disadvantage is present because, when X-ray film for direct use with a silver halide emulsion on both sides of a support is inserted between fluorescent screens and irradiated with X-rays, not only does the light emitted from one of the fluorescent screens form a latent image in the adjacent silver halide emulsion layer (becoming a developed blackened silver image), but a considerable amount of light passes through the support and also forms an indistinct latent image in the silver halide emulsion layer on the other side of the support. This phenomenon is known as "cross-over".
The degree of cross-over greatly affects the final image sharpness.
The image formed by cross-over is indistinct because the light is dispersed in the opposite silver halide emulsion layer and in the support. Additionally, the light is dispersed and refracted on the periphery of the opposite emulsion layer and support forming light reflections.
Much research has been done in the past with respect to the loss of image clarity due to cross-over and regarding the reduced sensitivity when the cross-over is eliminated as described in GB Patent No. 1,422,534, U.S. Pat. No. 3,989,527, GB Patent 504,283 and JP-A-54-3l737 and JP-A-49-69324, etc. (the term "JP-A" as used herein means an "unexamined published Japanese patent application").
All of the above methods have been unsatisfactory in practical use: some are effective in cutting out cross-over but reduce the sensitivity, other methods use silver which does not contribute to sensitivity, this being undesirable in times when silver is being economized, or still other methods use fluorescent substances which have a severe effect on photographic performance.
A further method of eliminating cross-over, as described by JP-A-58-l2792l and U.S. Pat. Nos. 4,416,986 and 4,413,053 employs the silver halide grains themselves to absorb the cross over light by enlarging the projected surface area of the photosensitive silver halide grains themselves.
In other words, a method has been disclosed for eliminating cross-over without increasing the amount of silver used (in proportion to the volume of silver halide grains) using tabular silver halide grains whose grain diameter is not less than 5 times the grain thickness. By this method, cross-over is eliminated without marked reduction of sensitivity and the sharpness is improved.
Now, with X-ray film for direct use, the usual procedure is that after development, the images on the X-ray film for direct use are read from the top of a light table, etc. through a light source.
However, when the surface of the blackened silver image is smooth and the image is observed from the top of a light table, etc., light emitted from a source behind the observer, e.g. a light in the room, is reflected from the blackened silver image. The observer's figure is thus reflected from the film as in a mirror, making it difficult to interpret the actual image. This problem is particularly prevalent in the material of JP-A-58-l2792l and in other direct X-ray films, in that the blackened silver image portion of the film for direct X-ray using tabular silver halide grains has a high reflection index, making it difficult to interpret the image.
Examples of methods for reducing the surface reflection index are disclosed in JP-A-57-l04l33, JP-A-57-20731, and JP-A-58-l63936 etc. Although the reflection index of the blackened silver image portion is thereby reduced, there are the disadvantages of a low image sharpness results and less light passes through non-image areas so as to increase the haze. Hence, it has been very difficult to simultaneously satisfy the requirements of sharpness, reflection index of the blackened silver image portion and haze of the non-image areas.
A further method for reducing the surface reflection index is described in JP-A-6l-20l235. Although the reflection index of the blackened silver image portion is thereby reduced, at least one additional layer of spherical grains or potato-shaped grains or silver halide grains having a diameter less than 5 times as great as the grain thickness, and which are less efficient in their use of silver than tabular silver halide grains, must be placed outside the emulsion layer comprising the tabular silver halide grains. As a result, more silver must be applied per unit area of the film to achieve the same maximum density (D.sub.max) as compared to film comprising only tabular silver halide grains, this increasing the manufacturing cost thereof.